Recent Advances in Chromium(II)- and Chromium(III)-Mediated Organic Synthesis

Abstract

Synthetic transformations utilizing chromium(II) or chromium(III) reagents, mainly CÐC-coupling reactions, are discussed. Chromium reagents find increasing application in complex total syntheses where other organometallics are difficult to apply. They are easy to prepare and exhibit extraordinary chemoselectivity and high diastereoselectivity. In this article emphasis is laid on recent results in synthetic procedures, on little known general aspects of (organo)chromium chemistry, and on areas not reviewed before. The most important recent advances include reactions catalytic in chromium ions, anti -selective Reformatsky-type aldol reactions with excellent asymmetric induction, domino radical/carbanion reactions, asymmetric chromium(III) catalyzed epoxide openings and homogeneous alkene polymerization catalysts. Some progress is also made in ligand controlled enantioselective reactions with Cr(II) reagents, but satisfying solutions remain a major challenge in the field.

Involvement of an Oxidation‐Reduction Equilibrium in Chromium‐Mediated Enantioselective Nozaki–Hiyama Reactions

“…[3,10 ± 18] Interestingly, an OMPV-reaction also seems to participate in other chromium(II)-mediated reactions like the Nozaki±Hiyama À Kishi and chromium-Reformatsky reactions. [3,5,8] In accordance with this observation, OMPV equilibria are especially relevant in products from substituted allylchromiums, whereas good enantioselectivities were initially reported for the unsubstituted case only. This prompted us to look more closely into the relationship between enantioselective Nozaki±Hiyama reactions and OMPV transformations.…”

“…[1,2] An elegant method for the synthesis of homoallyl alcohols is the chromium(II)-promoted Nozaki±Hiyama reaction, which allows the coupling of allyl halides with aldehydes in a Barbier-type reaction. [3,4] In our previous studies on the Nozaki±Hiyama reaction we observed that allyl ketones 4 can be common side products, or even the main product, although they are neglected by most authors. [5] This formation can be explained by an Oppenauer-type oxidation mechanism (Scheme 1) and is strongly favoured with sterically unhindered aromatic aldehydes and a substituted allylchromium.…”

“…Also, in crossover experiments, in which anisaldehydes were used to form the Hiyama±Nozaki-intermediates 3, followed by excess benzaldehyde as oxidant, only the anisaldehyde-derived products were detected and not a mixture reflecting the proportions of the two aldehydes applied. [5] More extensive studies on the inhibition of retro-reactivity in Cr(III) complexes [3] will be published elsewhere.…”

Trace Amounts of Second Metal Elements Can Play a Key Role in the Generation of Organometallic Compounds

“…31 Second, the rate of hydrolysis of chromiumcarbon bonds is relatively slow, in sharp contrast to the other metalcarbon bonds of early transition metals. 32 Therefore, addition reactions of organochromium reagents to carbonyl compounds do not require strictly anhydrous conditions. By taking this as an opportunity, nucleophilic organochromium reagents could be generated by addition of an equimolar amount of water; though slow addition of the water is required (Scheme 19).…”

“…13,32,34 1.3 Geminal Dimetallic Reagents for Olefination of Carbonyl Groups: Organotitanium Reagents.…”